Diabetes mellitus currently afflicts at least 200 million people worldwide. Type 1 diabetes accounts for about 10% of this number, and results from autoimmune destruction of insulin-secreting β-cells in the pancreatic islets of Langerhans. Survival depends on multiple daily insulin injections. Type 2 diabetes accounts for the remaining 90% of individuals affected, and the rate of prevalence is increasing. Type 2 diabetes is often, but not always, associated with obesity, and although previously termed late-onset or adult diabetes, is now increasingly manifest in younger individuals. It is caused by a combination of insulin resistance and inadequate insulin secretion.
In a non-stressed normal individual, the basal glucose level will tend to remain the same from day to day because of an intrinsic feedback loop. Any tendency for the plasma glucose concentration to increase is counterbalanced by an increase in insulin secretion and a suppression of glucagon secretion, which regulate hepatic glucose production (gluconeogenesis and release from glycogen stores) and tissue glucose uptake to keep the plasma glucose concentration constant. If the individual gains weight or becomes insulin resistant for any other reason, blood glucose levels will increase, resulting in increased insulin secretion to compensate for the insulin resistance. Therefore the glucose and insulin levels are modulated to minimize changes in these concentrations while relatively normal production and utilization of glucose are maintained.
Five different phases of insulin secretion have been identified: (1) basal insulin secretion wherein insulin is released in the postabsorptive state; (2) the cephalic phase wherein insulin secretion is triggered by the sight, smell and taste of food, before any nutrient is absorbed by the gut, mediated by pancreatic innervation; (3) first-phase insulin secretion wherein an initial burst of insulin is released within the first 5-10 minutes after the β-cell is exposed to a rapid increase in glucose, or other secretagogues; (4) second-phase insulin secretion wherein the insulin levels rise more gradually and are related to the degree and duration of the stimulus and (5) a third-phase of insulin secretion that has only been described in vitro. During these stages, insulin is secreted, like many other hormones, in a pulsatile fashion, resulting in oscillatory concentrations in the blood. Oscillations include rapid pulses (occurring every 8-15 minutes) superimposed on slower oscillations (occurring every 80-120 minutes) that are related to fluctuations in blood glucose concentration.
Insulin secretion can be induced by other energetic substrates besides glucose (particularly amino acids) as well as by hormones and drugs. Of note is that the insulin response observed after food ingestion cannot be accounted for solely by the increase in blood glucose levels, but also depends on other factors such as the presence of free fatty acids and other secretagogues in the meal, the neurally activated cephalic phase and gastrointestinal hormones.
When an individual is given an intravenous glucose challenge, a biphasic insulin response is seen which includes a rapid increase with a peak, an interpeak nadir and a subsequent slower increasing phase. This biphasic response is only seen when glucose concentration increases rapidly, such as after a glucose bolus or glucose infusion. A slower increase in glucose administration, what is seen under physiologic conditions, induces a more gradually increasing insulin secretion without the well-defined biphasic response seen in response to bolus infusion of glucose.
Modeling of first-phase insulin responses under normal physiologic conditions has demonstrated that, after a meal, glucose concentration increases more gradually (Cmax reached in approximately 20 minutes) than seen with intravenous bolus injections of glucose (Cmax reached in approximately 3-10 minutes).
Healthy pancreatic β-cells generate an early response to a meal-like glucose exposure that rapidly elevates serum insulin both in the portal circulation and in the periphery. Conversely, defective β-cells, which have an impaired first-phase insulin response, generate a sluggish response to the meal-like glucose exposure.
Increasingly, evidence indicates that an early relatively rapid insulin response following glucose ingestion plays a critical role in the maintenance of postprandial glucose homeostasis. An early surge in insulin concentration can limit initial glucose excursions, mainly through the inhibition of endogenous glucose production. Therefore the induction of a rapid insulin response in a diabetic individual is expected to produce improved blood glucose homeostasis.
In a normal individual, a meal induces the secretion of a burst of insulin, generating a relatively rapid spike in serum insulin concentration that then decays relatively quickly (see FIG. 1). This early-phase insulin response is responsible for the shut-off of release of glucose from the liver. Homeostatic mechanisms then match insulin secretion (and serum insulin levels) to the glucose load. This is observed as a slow decay of modestly elevated serum insulin levels back to baseline and is second-phase kinetics.
Type 2 diabetics typically exhibit a delayed response to increases in blood glucose levels. While normal individuals usually begin to release insulin within 2-3 minutes following the consumption of food, type 2 diabetics may not secrete endogenous insulin until blood glucose begins to rise, and then with second-phase kinetics, that is a slow rise to an extended plateau in concentration. As a result, endogenous glucose production is not shut off and continues after consumption and the patient experiences hyperglycemia (elevated blood glucose levels).
Loss of eating-induced insulin secretion is one of the earliest disturbances of β-cell function. While genetic factors play an important role, some insulin secretory disturbances seem to be acquired and may be at least partly reversible through optimal glucose control. Optimal glucose control via insulin therapy after a meal can lead to a significant improvement in natural glucose-induced insulin release by requiring both normal tissue responsiveness to administered insulin and an abrupt increase in serum insulin concentrations. Therefore, the challenge presented in treatment of early stage type 2 diabetics, those who do not have excessive loss of β-cell function, is to restore the rapid increase in insulin following meals.
In addition to the loss of first-phase kinetics, early stage type 2 diabetics do not shut-off endogenous glucose release after a meal. As the disease progresses, the demands placed on the pancreas further degrades its ability to produce insulin and control of blood glucose levels gradually deteriorates. If unchecked, the disease can progress to the point that the deficit in insulin production approaches that typical of fully developed type 1 diabetes. However, type 1 diabetes can involve an early “honeymoon” stage, following an initial crisis, in which insulin is still produced but defects in release similar to early type 2 disease are exhibited.
Most early stage type 2 diabetics are currently treated with oral agents, but with limited success. Subcutaneous injections are also rarely ideal in providing insulin to type 2 diabetics and may actually worsen insulin action because of delayed, variable and shallow onset of action. It has been shown, however, that if insulin is administered intravenously with a meal, early stage type 2 diabetics experience the shutdown of hepatic glucose release and exhibit increased physiologic glucose control. In addition their free fatty acids levels fall at a faster rate that without insulin therapy. While possibly effective in treating type 2 diabetes, intravenous administration of insulin, is not a reasonable solution, as it is not safe or feasible for patients to intravenously administer insulin at every meal.
Significant pathology (and morbidity) in diabetics is associated with inadequate control of blood glucose. Excursions of blood glucose concentration both above and below the desired, normal range are problematic. In treatments that fail to mimic physiologic insulin release, the rise in insulin concentration does not produce sufficiently high glucose elimination rates to completely respond to the glucose load resulting from a meal. This can be further exacerbated by failure to shut off glucose release from the liver. Additionally, with many forms of insulin therapy, serum insulin levels and glucose elimination rates also remain elevated after the prandial glucose load has abated, threatening hypoglycemia. Attempts to better control peak glucose loads by increasing insulin dose further increase this danger. Indeed, postprandial hypoglycemia is a common result of insulin therapy often causing, or even necessitating, patients to eat snacks between meals, depending on the severity of hypoglycemia. This contributes to the weight gain often associated with insulin therapy. These risks and their frequency and severity of occurrence are well understood in the art.
Current insulin therapy modalities can supplement or replace endogenously-produced insulin to provide basal and second-phase-like profiles but do not mimic first-phase kinetics (see FIG. 2). Additionally, conventional insulin therapy often involves only one or two daily injections of insulin. However, more intensive therapy such as three or more administrations a day, providing better control of blood glucose levels, are clearly beneficial (see for example Nathan, D. M., et al., N Engl J Med 353:2643-53, 2005), but many patients are reluctant to accept the additional injections.
Until recently, subcutaneous (SC) injection has been the only route of delivering insulin to patients with both type 1 and type 2 diabetes. However, SC insulin administration does not lead to optimal pharmacodynamics for the administered insulin. Absorption into the blood (even with rapid acting insulin analogues) does not mimic the prandial physiologic insulin secretion pattern of a rapid spike in serum insulin concentration. Since the discovery of insulin, alternative routes of administration have been investigated for their feasibility in improving the pharmacodynamics of the administered insulin and improving compliance by reducing the discomfort associated with SC injections.
The alternative routes of insulin administration which have been evaluated in detail include the dermal, oral, buccal, nasal and pulmonary routes. Dermal insulin application does not result in reproducible and sufficient transfer of insulin across the highly efficient skin barrier. Effective oral insulin administration has not yet been achieved, primarily due to digestion of the protein and lack of a specific peptide carrier system in the gut. Nasal insulin application leads to a more rapid absorption of insulin across the nasal mucosa, however not with first-phase kinetics. The relative bioavailability of nasal administered insulin is low and there is a high rate of side effects and treatment failures. Buccally absorbed insulin also fails to mimic a first-phase release (Raz, I. et al., Fourth Annual Diabetes Meeting, Philadelphia, Pa., 2004).
Recently, pulmonary application of insulin has become a viable insulin delivery system. Some pulmonary insulin formulations in development provide faster appearance of insulin in the blood than typical subcutaneously delivered products (see FIG. 3), but apparently do not adequately reproduce all aspects of first-phase kinetics.
Therefore, a need exists for an insulin formulation which can mimic first-phase kinetics to provide physiologic postprandial insulin pharmacokinetics and pharmacodynamics for improved control of blood glucose levels.